In a cellular network, such as one employing orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA), each cell employs a base station that communicates with user equipment, such as a cell phone, a laptop, or a PDA, that is actively located within its cell. When the user equipment is first turned on, it has to do an initial cell search in order to be connected to the cellular network. This involves a downlink synchronization process between the base station and the user equipment wherein the base station sends a synchronization signal to the user equipment. The synchronization signal is typically known as the synchronization preamble in the IEEE 802.16e or synchronization channel (SCH) in the 3GPP WCDMA/HSDPA.
During initial cell search, the user equipment establishes timing and frequency offset parameters. Timing involves knowing where to sample the start of the synchronization frame and associated symbols. Frequency offset involves determining the mismatch between the controlling oscillator at the base station and the local oscillator in the user equipment.
Depending on the quality of the local oscillator, the frequency offset may be large and require considerable search time as well as additional algorithms to accommodate. This effect is exacerbated if the user equipment is moving at car or train speeds. In addition to timing and frequency considerations, some information that is specific to the initial cell, such as physical cell identification (Cell ID), has to be acquired. Since downlink synchronization involves several operations, the design and procedure of downlink synchronization shall attempt to minimize the receiver complexity and time required for cell search. To aid the complexity reduction, the synchronization signal may consist of two portions: the primary and secondary synchronization signals. The primary signal is typically used for timing and frequency acquisition whereas the secondary signal is typically used to acquire the Cell ID and other cell-specific information. Unlike the secondary signal, the primary signal is typically common to all cells. The primary synchronization signal carries the primary synchronization sequence. To ensure competitive performance, the primary synchronization signal is used to obtain the channel estimates necessary for decoding the cell-specific information in the secondary signal via coherent detection.
As the moving user equipment approaches a cell boundary between two adjoining cells, it performs a neighboring cell search in preparation to handover its activation from the initial cell to the neighboring cell. During this time, it receives information from two or more base stations. When the base stations employ a common primary sequence, this common signal causes a mismatch between the channel experienced by the cell-specific transmissions and the transmitted primary signal for the user equipment. This mismatch is severe especially for terminals at the cell edges where each of the terminals receives two equally strong and overlapping channels from two significant base stations. Another problem associated with a common primary synchronization sequence is the timing mismatch between the channel experienced by the primary sequence and the cell-specific data transmission. In this case, the timing obtained from the primary sequence may result in performance degradation when used to demodulate a cell-specific data transmission. This phenomenon occurs especially in a tightly synchronized network, such as those deployed in the USA and Japan, and has become increasingly popular with medium to large cell radius. In addition, advanced cellular OFDM systems such as the 3GPP E-UTRA (enhanced UMTS Terrestrial Radio Access) or Long-term Evolution (LTE) accommodate the use of single frequency network (SFN) for the enhanced multimedia broadcast and multicast systems (E-MBMS) which heavily relies upon network synchronization. While this phenomenon is also relevant to the initial cell search, it is particularly problematic for the neighboring cell search as the operating signal-to-noise ratio (SNR) for the neighboring cell search is considerably lower. This performance reduction translates to larger cell search time, which may result in higher disconnect probability upon handover.
Accordingly, what is needed in the art is an enhanced way to accomplish initial and neighboring cell searches.